During the lifetime of a patient, it may be necessary to perform a joint replacement procedure on the patient as a result of, for example, disease or trauma. For example, many knee replacement surgeries are performed each year. Total knee replacement or arthroplasty may involve replacement of the mid-shaft portion of the femur, proximal, distal, and/or total femur, and proximal tibia. Unicompartmental knee replacement or arthroplasty involves unicondylar resurfacing. Unicompartmental knee arthroplasty provides an alternative to total knee arthroplasty for rehabilitating knees when only one condyle has been damaged as a result of trauma or disease such as noninflammatory degenerate joint disease or its composite diagnosis of osteoarthritis or post-traumatic arthritis. As such, unicompartmental knee arthroplasty may be indicated for use in patients undergoing surgery for a severely painful and/or disabled joint damaged as a result of osteoarthritis, traumatic arthritis, rheumatoid arthritis, or a failed previous implant when only one condyle of the knee (medial or lateral) is affected. Further, unicompartmental knee replacements may be “multi-piece” replacements in which a unicompartmental tibial insert is used to replace each of the medial and lateral condyles of the patient. A single, total femoral component or two partial femoral components may be used to cooperate with the two unicompartmental inserts.
In addition, in some knee replacement procedures, a total knee tibial tray may used with a unicompartmental tibial insert. For example, a total knee tibial tray may be used with a single unicompartmental tibial insert to replace either the medial or lateral condyle of the patient's knee. Alternatively, a total knee tibial tray may be used with two unicompartmental tibial inserts, each replacing one of the medial and lateral condyles of the patient's knee. In such applications, the medial and lateral unicompartmental tibial inserts may have different characteristics and be selected based on the orthopaedic considerations associated with the respective condyle of the patient's knee.
Unicompartmental knee replacements are intended to provide increased patient mobility and reduce pain by replacing the damaged knee joint articulation in patients where there is evidence of sufficient sound bone to seat and support the components. Age and activity level factor into all reconstructive procedures and the state of the arthritis determines the treatment. With the advancement of minimally invasive techniques that support unicompartmental knee reconstruction, a growing number of patients are offered this alternative for relief from the disabling pain of arthritis and for the potential benefits of a rapid recovery.
A tibial assembly of a unicompartmental knee prosthesis typically includes a tibial tray configured to be coupled to the patient's tibia and a polymer tibial bearing or insert adjacent the tibial tray. As discussed above, the tibial tray may be a total or unicompartmental tibial tray. The tibial insert includes an upper bearing surface configured to engage a corresponding articulating condylar surface of a femoral component coupled to the patient's femur. A mobile tibial assembly generally refers to a tibial assembly wherein the tibial insert is movable relative to the tibial tray. In other words, the tibial insert may rotate relative to the tray and/or the tibial insert may move medially, laterally, anteriorly, and/or posteriorly relative to the tibial tray. This motion of the tibial insert relative to the tray may be constrained in any number of ways in order to limit the type of motion of the tibial insert. For example, the tibial insert may be limited to anterior/posterior motion relative to the tibial tray and/or rotation of the tibial insert relative to the tibial tray may be limited to something less than 360 degree rotation. A fixed tibial assembly generally refers to a tibial assembly wherein the tibial insert is not movable relative to the tibial tray and remains in a fixed location thereon. Surgeons may choose between fixed and mobile tibial assemblies depending upon the particular needs of the patient.
Typical mobile tibial assemblies fall into one of two classifications with respect to the insert-to-tray interface: unconstrained and constrained. In an unconstrained mobile tibial assembly, the tibial insert is free to move in all directions relative to the tibial tray. In a constrained mobile tibial assembly, the tibial insert is typically restricted from movement relative to the tibial tray in all but one or more directions and/or movements (e.g., translations and/or rotations).